In the optical information processing field and the optical communication field, it has been attempted to increase communication capability by optical multiplex communication. Wavelength multiplexing, which permits the transmission of a large number of wavelengths in one fiber, enables a drastic increase in the transmission capacity of the fiber. For the wavelength multiplexing transmission, optical routing plays an important role in demultiplexing and multiplexing light depending on wavelengths. To this end, signal light is controlled by converting light with a specific wavelength into light with a different wavelength. As a method of this optical routing, difference-frequency conversion is used, which utilizes a nonlinear optical effect. Signal light and pump light are introduced into a nonlinear optical element, and difference-frequency light between the signal light and the pump light is rendered new signal light, whereby wavelengths of the signal light can be converted collectively. Since the nonlinear optical effect is utilized, wavelength conversion at a high speed is possible. As such a wavelength conversion element, a waveguide-based difference-frequency generation device that utilizes quasi phase matching has been proposed (see M. H. Chou, et al., OPTICS LETTERS, 1998, vol. 23, No. 13, pp 1004–1006).
FIG. 10 shows the configuration of a conventional optical waveguide-based difference-frequency generation device. On a LiNbO3 substrate, a periodically domain-inverted structure 801 and a proton-exchanged waveguide 802 are formed. At an incident portion of the optical waveguide, a segment tapered waveguide 803 is formed. Light with a wavelength in 1.56 μm-band is used as signal light, light with a wavelength of 0.78 μm is used as pump light and light with a wavelength in 1.56-μm band is used as difference-frequency light. In order to satisfy the condition for allowing the 1.56-μm signal light and difference-frequency light to be guided, the optical waveguide has a multi-mode condition for the pump light with a wavelength of 0.78 μm. In this regard, it is difficult to couple light in a single mode with a proton-exchanged waveguide having a multi-mode condition. For that reason, incident portions are provided separately for the signal light and the pump light, and the segment tapered waveguide 803 is used for the incident portion of the pump light.
The segment tapered waveguide 803 offers a single mode condition to the pump light at the incident portion of the waveguide, and then gradually introduces the guided light into the optical waveguide having a multi-mode condition, so as to let the guided light propagate in a single mode through the multi-mode waveguide. In other words, the segment tapered waveguide allows the conversion in the waveguide from the single-mode propagation light in the single-mode waveguide to the single-mode propagation light in the multi-mode waveguide. The signal light and the pump light are allowed to propagate in the single mode through the optical waveguide having the domain-inverted structure, whereby overlap in the waveguide increases, thus enabling the generation of difference-frequency light with high efficiency.
However, the optical waveguide used in a conventional optical waveguide device is a stripe-shaped three-dimensional waveguide, which does not have a symmetrical configuration in refractive index distribution. Therefore, the following problems occur: it is significantly difficult to externally excite the single mode only in the multi-mode waveguide; and the tolerance is considerably narrow. This is because unless the electric field distribution of a beam spot of the externally incident light agrees with the electric field distribution of the single mode of the optical waveguide accurately, the excitation of multi-mode occurs easily. General lens coupling and optical fiber coupling are incapable of selectively exciting the single mode only in the multi-mode waveguide.